Abstract: A self-referenced optical fiber cure sensor is proposed to in-situ monitor the cure of epoxy resins. The sensor is fabricated by cleaving a multimode step index optical fiber whose core is of refractive index 1.558, the cleaved end is used to infer the refractive index of the epoxy during cure. The refractive index of the fiber core serves as a reference to calibrate the zero bias and sensitivity of the sensor. As the refractive index of the epoxy system increases with the extent of cure, it will equal the refractive index of the fiber core at a cure extent and then the calibration can be done. Thus, in the later parts of the cure, the sensor can provide quantitative information about cure reaction. When monitoring the cure of composites, the sensors embedded in different part of the structure can be normalized to a common scale such that the real-time comparison of each sensor's output becomes possible. The validity and feasibility of the proposed sensor have been confirmed in the cure monitoring experiments on an epoxy system.